I like this scatter plot. If you really want to get freaky with it, you should take into account the “protein digestibility-corrected amino acid scores.” Things like eggs and whey are 100%, beans are usually in the high 70’s, and peanuts are actually down at near 50%.

So for nutrition’s sake, not all protein sources are created equal, and it makes sense that if you are trying to get adequate protein at the lowest price, you also want to get sources where you can eat the least of it to satisfy the protein requirements of your body.